The present invention pertains to communications systems using orthogonal frequency division modulation to communicate information from a mobile unit.
Multi-carrier modulation systems generally involve a data signal of successive symbols being split into several lower rate signals, each associated with a sub-carrier, resulting in a large symbol time compared to the expected multipath delay spread. Orthogonal frequency division modulation (OFDM) is a multi-carrier modulation scheme that maps data symbols onto N orthogonal sub-carriers separated by a distance of 1/T, where T is the useful symbol duration. In OFDM, cyclic guard intervals are frequently used to improve performance in the presence of a multipath channel.
OFDM has become attractive for wireless communications due to its high spectral efficiency and resistance to noise and multipath effects. OFDM has been the subject of numerous patents and at the foundation of a number of wireless broadcast standards, including for example the ETSI-monitored DAB and DVB-T protocols. Both of these standards provide for single frequency network (SFN) operation in which a number of transmitters operate in simulcast manner. As apparent from the DAB and DVB-T protocols, much of the focus to date on OFDM has been in respect of providing broadband, high speed downstream transmission (ie. transmission from base or repeater stations to remote wireless units). Although the concept of providing upstream data (ie. transmission of information from remote wireless units to a base station) has been considered in the context of OFDM systems, such systems have generally assumed that the transfer of data from a remote wireless unit to a base station will generally require lower speeds and less bandwidth than downstream communications.
In some applications, however, high speed, large bandwidth transfer of data from a remote unit to a central station is required. For example, mobile electronic news gathering (ENG) systems, in which audio and video signals from a news van, helicopter or other mobile vehicle are transmitted to a central news gathering facility, require high data rate upstream communications. Furthermore, as such systems are often used in high density urban areas, they require robust, multipath resistant, communications channels. When the mobile units are moving while transmitting, the channel must also be resistant to the effects of Doppler spread.
Thus, there is a need for an OFDM communications system that provides for robust, economical, high data rate wireless transfer from a remote mobile transmitter to a central station. An electronic news gathering system having these features is also desirable.